When a helicopter hovers, each of the blades is moving at the same speed relative to the air (i.e., each blade has the same air speed). Thus, each blade generates the same amount of lift. However, as a helicopter moves, the helicopter blade or blades that are moving in the same direction as the helicopter movement (i.e., the advancing blades) experience a greater air speed, and the helicopter blade or blades that are moving in the opposite direction as the helicopter movement (i.e., the retreating blades) experience a lower air speed. In such a case, the advancing blades generate more lift and flap up relative to their hover positions, and the retreating blades generate less lift and flap down relative to their hover positions. This results in a phenomenon known as dissymmetry of lift where the advancing blades generate more lift than the retreating blades.
To compensate for dissymmetry of lift and provide a stable helicopter, helicopter rotors are commonly designed to accommodate at least some amount of up and down flapping motion while changing the angle of attack for each of the blades. For instance, a helicopter rotor can be designed to decrease the angle of attack of an advancing blade as it flaps up relative to its hover position, thereby decreasing the amount of lift that is generated. Similarly, the helicopter rotor can be designed to increase the angle of attack of a retreating blade as it flaps down relative to its hover position, thereby increasing the amount of lift that is generated. Accordingly, the combination of flapping and changing the angles of attack of the blades can be used to balance the lift generated by each of the blades.